I had a check engine light come on saying misfire on cylinder one. After having a pretty thorough inspection done the concensus was a bad cylinder one fuel injector so I had that replaced. On the way back home from the shop the check engine light came back on. This time it was that it was running to lean from bank 1. This was from the pre-cat O2 sensor. Soooo, we cleaned the MAF sensor and throttle body, checked for vacuum leaks and replaced the pre-cat O2 sensor. The check engine light came back on after about 15 miles of driving with the same running to lean message. What do you think it is? FYI, 1996 T100 4x4 5 speed.

I would check all the rubber intake tubing connectors carefully for cracks that could cause leaks. And I would also double check in the FSM that you replaced the correct O2 sensor. If bank 0 is the front one, bank 1 might be the rear. I don't remember how they're numbered offhand...

I'm pretty darn certain the code was for the pre-cat sensor and the intake was checked for leaks so if we rule those two items out, where would you look next? Could it be that the MAF has gone bad? Maybe a defective fuel injector was installed? The truck is running great. Any other thoughts?

What about some part of the evap system? Could be something silly like the charcoal canister or PCV ... <img src="/forums/images/graemlins/confused.gif" alt="" />

The front o2 sensor is for regulation and the rear o2 sensor is for monitoring. OBD2 vehicles are required to monitor cat efficiency.

So, the lean condition is being detected by the front o2 sensor.

Unless you are dropping codes related to cat efficiency (P0420) then it is my opinion that replacing the rear o2 sensor is not worthwhile.

Both o2 sensors on the 3.4 are in "bank 1." Sensor 1 is in front, Sensor 2 is in the rear.

I had a code for cyl 1 misfire a long time ago. The truck also ran rough. I replaced the ignition wires and the CEL only occurs once in a great while and goes away when I pull the ECU fuse for a few minutes. It doesn't recur for months. And it only seems to happen when I reall stomp on the accelerator.

The rear O2 sensor is often used for 'fuel trim.' Cannot speak for the T100 but on many vehicles the rear sensor does play into A/F ratio in addition to its primary purpose as the catalyst diagnostic. The front sensor usually degrades sooner so the rear sensor corrects for 'drift' in the primary [front] sensor signal.

It is unlikely though that the rear sensor would go bad without triggering a cat code or a dedicated rear sensor code. 'Lean' fuel codes are usually just that - the engine is running lean. You could pull the plugs and look for color differences to see if it is injector/cylinder specific.